The present invention relates to electromagnetic interference (xe2x80x9cEMIxe2x80x9d) shields and, more specifically, to EMI finger shields forming nonlinear slits between adjacent fingers, whether the shields are manufactured from metal or from an electrically nonconductive material and clad with an electrically conductive layer.
During normal operation, electronic equipment generates undesirable electromagnetic energy that can interfere with the operation of proximately located electronic equipment due to EMI transmission by radiation and conduction. The electromagnetic energy can by of a wide range of wavelengths and frequencies. To minimize the problems associated with EMI, sources of undesirable electromagnetic energy may be shielded and electrically grounded. Shielding is designed to prevent both ingress and egress of electromagnetic energy relative to a housing or other enclosure in which the electronic equipment is disposed. Since such enclosures often include gaps or seams between adjacent access panels and around doors, effective shielding is difficult to attain because the gaps in the enclosure permit transference of EMI therethrough. Further, in the case of electrically conductive metal enclosures, these gaps can inhibit the beneficial Faraday Cage Effect by forming discontinuities in the conductivity of the enclosure which compromise the efficiency of the ground conduction path through the enclosure. Moreover, by presenting an electrical conductivity level at the gaps that is significantly different from that of the enclosure generally, the gaps can act as slot antennae, resulting in the enclosure itself becoming a secondary source of EMI.
Specialized EMI gaskets have been developed for use in gaps and around doors to provide a degree of EMI shielding while permitting operation of enclosure doors and access panels. To shield EMI effectively, the gasket should be capable of absorbing or reflecting EMI as well as establishing a continuous electrically conductive path across the gap in which the gasket is disposed. Conventional metallic gaskets manufactured from copper doped with beryllium are widely employed for EMI shielding due to their high level of electrical conductivity. Due to inherent electrical resistance in the gasket, however, a portion of the electromagnetic field being shielded induces a current in the gasket, requiring that the gasket form a part of an electrically conductive path for passing the induced current flow to ground. Failure to ground the gasket adequately could result in radiation of an electromagnetic field from a side of the gasket opposite the primary EMI field.
In addition to the desirable qualities of high conductivity and grounding capability, EMI gaskets should be elastically compliant and resilient to compensate for variable gap widths and door operation, yet tough to withstand repeated door closure without failing due to metal fatigue. EMI gaskets should also be configured to ensure intimate electrical contact with proximate structure while presenting minimal force resistance per unit length to door closure, as the total length of an EMI gasket to shield a large door can readily exceed several meters. It is also desirable that the gasket be resistant to galvanic corrosion which can occur when dissimilar metals are in contact with each other for extended periods of time. Low cost, ease of manufacture, and ease of installation are also desirable characteristics for achieving broad use and commercial success.
Conventional metallic EMI gaskets, often referred to as copper beryllium finger strips, include a plurality of cantilevered or bridged fingers forming linear slits therebetween. The fingers provide spring and wiping actions when compressed. Other types of EMI gaskets include closed-cell foam sponges having metallic wire mesh knitted thereover or metallized fabric bonded thereto. Metallic wire mesh may also be knitted over silicone tubing. Strips of rolled metallic wire mesh, without foam or tubing inserts, are also employed.
One problem with metallic finger strips is that to ensure a sufficiently low door closure force, the copper finger strips are made from thin stock, for example on the order of about 0.05 mm (0.002 inches) to about 0.15 mm (0.006 inches) in thickness. Accordingly, sizing of the finger strip uninstalled height and the width of the gap in which it is installed must be controlled to ensure adequate electrical contact when installed and loaded, yet prevent plastic deformation and resultant failure of the strip due to overcompression of the fingers. To enhance toughness, beryllium is added to the copper to form an alloy; however, the beryllium adds cost. Finger strips are also expensive to manufacture, in part due to the costs associated with procuring and developing tooling for outfitting presses and rolling machines to form the complex contours required. Changes to the design of a finger strip to address production or performance problems require the purchase of new tooling and typically incur development costs associated with establishing a reliable, high yield manufacturing process. Notwithstanding the above limitations, metallic finger strips are commercially accepted and widely used. Once manufacturing has been established, large quantities of finger strips can be made at relatively low cost.
Metallic mesh and mesh covered foam gaskets avoid many of the installation disadvantages of finger strips; however, they can be relatively costly to produce due to the manufacturing controls required to realize acceptable production yields.
Another problem with conventional finger strips is that they are not as effective in EMI shielding as clock speed of an electronic product is increased. As clock speed is increased, the wavelength of the EMI waves produced decreases. Accordingly, the waves can penetrate smaller and smaller apertures in the enclosure and in the EMI shield. At lower wavelengths, the slits formed in the finger shields can act as slot antennae, permitting the passage of EMI therethrough and the resultant shielding effectiveness of the shields decreases. Conventional finger strips with linear slits formed between the fingers are increasingly less effective in these applications.
A metallized fabric clad polymer EMI shield overcomes many of the limitations and disadvantages of conventional EMI shields. One method of manufacturing a metallized fabric clad polymer shield for shielding EMI from passing through a seam between first and second electrically conductive bodies includes forming a base and a profile of an electrically nonconductive solid material in a predetermined configuration. The base is designed to secure the shield to the first body while the profile is designed to contact the second body. An electrically conductive layer is then disposed on at least part of the profile so as to be interdisposed between the profile and the second body upon installation of the shield in a suitable gap of an electronic enclosure. In one exemplary embodiment, the profile and base may be an extrusion of a polymer such as polyvinyl chloride (xe2x80x9cPVCxe2x80x9d), a thermoplastic resin, and the conductive layer may be a metallized fabric bonded to the profile by a heat sensitive glue. The forming and deposition processes may be separate or may be substantially contiguous. After extrusion and cooling of the profile and base, the metallized fabric may be bonded to the profile in a separate operation. Alternatively, by employing an in-line crosshead extrusion method, the polymer base and profile may be formed and substantially immediately thereafter, the metallized fabric applied as a thermally activated glue-backed tape. Resultant thermal energy in the extrusion activates the glue on the fabric side of the tape, bonding the metallized fabric to the profile. As a subsequent step in either manufacturing method, the profile may be divided into a plurality of independently flexible cantilevered or bridged fingers to compensate for variable gap width along the length of the gap.
Another embodiment for manufacturing a metallized fabric clad polymer EMI shield according to the invention includes disposing an electrically conductive layer on an electrically nonconductive solid sheet material and then forming the sheet into a base and a profile of a predetermined configuration. The sheet may be a polymer such as PVC, the conductive layer may be a metallized fabric bonded to the sheet by a thermally activated glue, and the profile and base may be formed by a thermal process such as thermoforming. As a subsequent step in the manufacturing method, the profile may be divided into a plurality of independently flexible cantilevered or bridged fingers.
According to certain embodiments of the invention, a metallized fabric clad polymer shield for shielding EMI from passing through a seam between first and second electrically conductive bodies includes a base for securing the shield to the first body, a profile of an electrically nonconductive solid material attached to the base for contacting the second body, and an electrically conductive layer disposed on the profile.
The base and the profile may be formed integrally of the same material by extrusion or of different materials by co-extrusion. Alternatively, the base and the profile may be similar or distinct materials joined together by bonding. Additionally, a hinge of a material exhibiting different flexural characteristics may be disposed between the base and the profile, either by co-extrusion or bonding. As used herein, the term bonding includes chemical processes such as those using glues or solvents, as well as mechanical processes such as friction welding and interlocking mechanical cross-sections.
To facilitate installing the EMI shield in an enclosure, an adhesive strip may be attached to the base. Alternatively, the base may include apertures for mechanical fasteners or a return for insertion in a slot or for capturing a flange of the first body. The return may also include barbing to stabilize the shield once installed.
Further EMI shield effectiveness may be achieved using nonlinear shaped EMI shielding fingers forming nonlinear slits therebetween. Various embodiments of nonlinear fingers and slits are contemplated, including those which are arcuate, sinusoidal, interdigitated, chevron-shaped, and combinations thereof. By using nonlinear fingers, the slits formed between the fingers do not form linear apertures so that EMI transmission in a given direction cannot line up with an entire slit length, but only a limited portion of the slit length. For arcuate and sinusoidal slits, as the radius of curvature is decreased, in the limit, the wavelength of EMI waves capable of passing through the slits approaches slit width, which can be substantially zero.
In one embodiment, the EMI shielding fingers with nonlinear slits may be manufactured out of an electrically conductive material such as a copper beryllium alloy or other metallic material. In another embodiment, the NM shielding fingers with nonlinear slits may be manufactured out of a metallized fabric clad polymer or other coated or clad material composition. In general, the nonlinear slits may be employed in any EMI finger shield, regardless of the material and method of manufacture, providing greatly improved EMI shielding effectiveness for short and very short wavelength radiation.